The present invention relates generally to optical information processing, and more particularly to optical automatic gain control using stable, non-absorbing optical hard limiters.
In today""s information age, optical communication technologies are being used more and more frequently for transmitting information at very high speeds. Traditionally, information processing equipment (such as switches, routers, and computers) process information electronically. Therefore, optical communications are often converted into electronic form for processing by the information processing equipment. This electronic processing is slow relative to the speed of the optical communications themselves, and thus often becomes a xe2x80x9cbottleneckxe2x80x9d of optical communication and processing systems.
A communication channel can be used more efficiently to transmit information if an encoding scheme is used to assign binary values to discrete intensity levels. This is difficult in an optical communication system due to the difficulty in controlling the intensity of the signal due to attenuation in the optical fiber. Therefore, it is difficult to establish a reference intensity level for optical communications over the optical fiber.
Automatic gain control can be used to normalize packets of varying intensities. Automatic gain control for optical communications is often accomplished by detecting the optical signal, transforming the optical signal in an electronic signal, processing the signal electronically, converting the processed electronic signal back into an optical form, and retransmitting the converted optical signal. Unfortunately, this process is limited by the speed of the electronics.
In accordance with one aspect of the present invention, optical automatic gain control (AGC) is accomplished using stable, non-absorbing optical hard limiters and various optical logic gates derived therefrom. The AGC mechanism preserves the ratios between signal levels and provides an adjustable amount of gain.
An optical automatic gain controller typically includes a number of AGC stages, where, in each AGC stage, a threshold input signal derived from an optical input signal is compared against a predetermined threshold for the AGC stage, and a gain input signal also derived from the optical input signal is amplified if and only if the threshold input signal is below the predetermined threshold. The threshold is reduced in each successive AGC stage.